* Final vote on defense spending could come Thursday
* Little chance of amendments passing
By David Alexander
WASHINGTON, May 25 (Reuters) - War-weary U.S. lawmakers pushed President Barack Obama to wind down the 10-year-old conflict in Afghanistan on Wednesday as the House of Representatives began debating a bill to authorize $690 billion in defense spending next fiscal year.
Republicans and Democrats aiming to ramp up pressure on the president introduced 18 amendments on Afghanistan, some demanding the start of a phased withdrawal and others seeking a radical shift away from the military's current troop-intensive counterinsurgency-style strategy.
The anti-war amendments had little chance of winning the 217 votes needed for passage if all members vote. Supporters were hopeful of topping the 162-vote high that similar measures have received in the past in order to send a message to Obama ahead of his decision on troop withdrawals in July.
"We're trying to put some wind at the president's back so that in July there will be more than just a token drawdown," said Democratic Representative Jim McGovern, who expressed concern about reports that only 5,000 service members might be withdrawn.
"It'll help empower the president to do what I think in his heart he knows is the right thing to do -- and that is to bring this war to an end," McGovern said.
Pressure to wind down the war came as the House began debating the bill that would authorize defense spending for the 2012 fiscal year, including a $571 billion base budget for the Pentagon and $119 billion for overseas contingencies, mainly the Iraq and Afghanistan wars.
A final vote on the bill could come as early as Thursday.
Although the bill authorizes expenditures, it is primarily a means for Congress and the administration to set defense policy. Actual spending levels are established by appropriations bills and other measures.
The bill being considered by the Republican-controlled House would impose restrictions on the Obama administration's effort to implement the New START nuclear arms treaty with Russia and undermine the repeal of a ban on gays serving openly in the military.
It also seeks to force the administration to continue several military programs eliminated to reduce costs at a time when the government is under pressure to cut its $1.4 trillion deficit and pay down its $14.3 trillion debt.
The House was considering more than 150 proposed amendments to the bill, dealing with everything from military use of alternative fuels and contractor oversight to the repatriation of remains of 13 sailors killed in the First Barbary War of 1804 and buried in a mass grave in Tripoli.
Many tried to influence U.S. war policy in Afghanistan, pressing for early withdrawals, ending aid to Pakistan or demanding a change in strategy.
"I have been for a long time under the impression that this counterinsurgency strategy does not work," said Republican Representative Walter Jones, who backs a proposal for a counter-terrorism approach requiring fewer troops.
"Let's do it exactly how we got bin Laden," he said. "Let's don't send 100,000 troops over there. Let's just find out where they are, let's send planes in and bomb the hell out of them."
Additional reporting by Susan Cornwell; Editing by Bill Trott